Generally, in mass spectrometer systems, atoms and molecules present in a subject sample are ionized, i.e., converted into ions, and introduced into a mass spectrometer where the ionic species are separated according to their mass-to-charge ratio. A charged-particle detector located at the exit of the mass spectrometer counts the separated ions in order to identify the mass and velocity distribution of ions in an ion beam. From this, information useful in determining the chemical composition of the sample can be determined.
Mass spectrometers are known in the prior art. However, these prior art systems exhibit several drawbacks. For example, many employ magnetic fields and therefore inherently require bulky magnets and shielding while others require precise machining and placements of the various mass spectrometer components such as multi-pole rods. Because of these requirements, prior art devices may not be easily miniaturized and therefore are not amenable to the next generation of millimeter and sub-millimeter-sized spectrometers. Such miniaturized and micro-machined instruments are desired and needed. Examples of use include pollution monitoring in factories, homes, auto exhausts, residual gas analysis and plasma processing, as well as and on spacecraft, for low-mass, low-power investigations of planetary environments.
As alluded to above, an important aspect of any spectrometer device or system is the ionization of the subject sample. In the spectrometric method described above, the ionization of the sample plays an important role and may dictate limitations on the use of the mass spectrometer device. Generally, there are two types of ionization techniques, typically referred to as “soft” ionization and “hard” ionization. The label of “soft” or “hard” refers to the degree of fragmentation of molecular ions in the ionization process. In “soft” methods, there is a minimum or negligible amount of ion fragmentation as opposed to “hard” methods where the degree of molecular ion fragmentation is much higher. Soft ionization methods often provide advantages over hard methods and are therefore desirable for many applications. For example, using soft ionization techniques, inorganic and organic compounds may be analyzed with the preservation of supramolecular assemblies.
Conventional prior art mass spectrometers often use “hard” techniques of producing ions, in which molecules are forcibly fractured. For example, ions may be produced by ultraviolet, radioactive, and/or thermal electron bombardment ionization techniques. As discussed above, these “hard” methods result in a significant degree of molecular ion fragmentation and may be undesirable in applications where the preservation of the molecular ion integrity is beneficial or even necessary.
Different mass spectrometer systems using ionization are known in the art. For example, quadrapole, magnetic sector and time of flight systems ionize sample material to determine its compositional content. Each of these devices have certain limitations in both operational use and size, however. For example, the quadrapole and magnetic section devices have relatively low resolution and therefore are limited in their compositional analyzing capabilities. These devices may also suffer from the standpoint of efficiency, especially during the ionization process.
Another disadvantage of prior art mass spectrometers utilizing hard ionization methods is that such systems generally require a high vacuum (10−5 Torr or better) environment. One reason for such a requirement is to enhance the life of the filament source. Secondly, and more importantly, a high vacuum environment is required so that ion collisions can be avoided during passage through the mass spectrometer device. With such rigorous demands, a vacuum pump must be provided to maintain a high vacuum under a variety of sample load conditions.
Vacuum pumps also consume power, may be heavy, large and typically require a relatively leak free environment. These limitations and necessities resulting from the need for a vacuum environment, and therefore the need for a vacuum pump, hinder the desired goal of instrument miniaturization.
Other desirable applications may be achieved and may benefit from the use of ionization systems if such systems were sufficiently small. However, existing ionization systems exhibit problems and difficulties in fabrication on smaller scales and therefore have not been suitable for use in these other desirable applications.
One particular form of mass spectrometer is disclosed in U.S. Pat. No. 5,726,448 to Smith et al. This device, called a rotating field mass spectrometer, provides a mass and velocity analyzer that utilizes time dependent electric fields to guide the ions being detected. Thus, precision ion beam apparatuses are not required.